Evidence for the Absence of Triphosphate Termini From the 5′ Ends of Newly Synthesized E.Coli DNA Molecules

Abstract

A picture of how E.coli DNA replicates is slowly coming into focus. There appears to be a specific origin of replication at around 82 minutes on the genetic map. From this origin replication is initiated once every generation and proceeds in both directions around the circular E.coli DNA; termination is presumed to occur when the two “growing forks” meet. The term “growing fork” is used to refer to that region of a replicating DNA molecule ahead of which the DNA has not been replicated and behind which the DNA has been replicated. A variety of proteins required for DNA replication have been identified by virtue of mutations in the genes coding for them (Wechsler, this volume), and several of these proteins have been highly purified and some of their enzymatic activities elucidated (Kornberg, this volume; Wickner, ibid). The bacterial chromosome is a circular double-stranded DNA molecule in a super-helical state (Wang, this volume) most likely in association with one or more “histone-like” proteins. During replication the two parental DNA strands must be unwound and separated, as it is known that DNA replication is semi-conservative. This requires repeated “nicking” and repair of the parental strands.